Measures to prevent future depletion of energy resources are being made intensively including movements to promote energy saving and recycling of resources, and development of technology to make these movements possible. In recent years, an intense effort is being made worldwide to lower CO2 emissions occurring from fuel combustion in order to prevent global warming.
Examples of internal combustion engines with low CO2 emissions include diesel engines used in automobiles. However, even though CO2 emissions are low, the diesel engine has a problem of black smoke emission. Black smoke occurs when there is not enough oxygen for the fuel being injected. That is, a dehydrogenation reaction occurs due to partial thermal decomposition of the fuel, producing a precursor to black smoke. This precursor thermally decomposes again, and agglomerates and coalesces, resulting in black smoke. This black smoke causes air pollution and adversely affects the human body.
Boosting the injection pressure of the fuel injected into the diesel engine combustion chamber can decrease black smoke. However, this requires the steel pipe used for fuel injection to have high fatigue strength. Examples of inventions related to the method for producing a steel pipe for this type of fuel injection include the following.
Patent document 1 discloses a method for producing a steel pipe for fuel injection in diesel engines where the inner surface of a hot rolled seamless steel pipe material is turned and polished by shot blasting, and then subjected to cold drawing. Using this production method reduces the depth of defects (irregularities, scab, tiny cracks, etc.) in the inner surface of steel pipe to within 0.10 mm, and therefore increases the strength of the steel pipe used for fuel injection.    [Patent document 1] JP H09-57329A
Although the steel pipe for fuel injection produced by the method disclosed in patent document 1 has high strength, the fatigue life does not match the strength of the steel pipe. Increasing the strength of the steel pipe material allows increasing the pressure load on the inner side of the steel pipe. However, the strength of the steel pipe material is not the only parameter that determines the internal pressure (hereinafter referred to as “internal pressure limit”) that serves as a limit below which no fatigue failure occurs when pressure is applied to the inner side of the steel pipe. In other words, the desired or higher internal pressure limit cannot be obtained just by increasing the strength of the steel pipe material. The fatigue life is preferably as long as possible considering the reliability of the end product, but if the internal pressure limit is low, then the steel pipe will be subject to fatigue in high internal pressure applications, resulting in shortened fatigue life.